An image sensor for an electronic camera ordinarily contains a two-dimensional array of light sensitive photosites. In an interline transfer sensor, the image charge is transferred from the photosites to light-protected vertical registers, through which the image charge is moved to a horizontal readout register. On the other hand, in a full frame image sensor the image charge is incremented line-by-line through the photosites themselves to a horizontal readout register, from which the image signal is obtained. Since the full frame image sensor does not contain a separate light-protected storage area, a shutter is normally used to block the light and prevent vertical smearing when reading out the image.
An example of an electronic still camera is the model DCS 200c camera, marketed by Eastman Kodak Company, Rochester, N.Y. This camera uses a high resolution full frame image sensor in an electronic camera back which attaches to a conventional 35 mm film camera body. The camera body includes a normal focal plane shutter for blocking image light. In this type of electronic camera, a separate focus sensor is used in the camera body to automatically focus the camera lens. This extra sensor increases the camera cost. It may also result in a less than optimally focused image on the sensor, if the tolerances of the camera lens focusing mechanism, and the tolerances of the focus detector, are not tightly controlled. Therefore, by using the high resolution image sensor itself to focus the camera lens, the camera cost may be reduced, and the camera focusing accuracy may be increased.
Focusing a camera lens by using the sensor output signal is an iterative process which requires capturing a sequence of images while varying the focus, until a focus-related parameter of the image, such as the "average contrast", is maximized. Prior art camcorders typically use NTSC format interline image sensors, which do not require a mechanical shutter, to perform this type of automatic lens focusing. In these camcorders, the same image that is recorded is also analyzed to provide the focus information. Focusing is done by spatially bandpass filtering a subsection of the video image read out from the sensor. The lens focus position is adjusted to obtain the highest average magnitude output signal (highest average contrast) from the bandpass filter. Note that the camcorder's image sensor is ordinarily read out at the video field rate (1/60 second) so that the same sensor operating mode is used to provide both the focus information and the final images. In such camcorders, many images may need to be read out before the lens is properly focused. Since the image readout time is relatively rapid (approximately 1/60 second), acceptable focus can typically be achieved in less than a second.
Focus time can be further improved by application of the technique shown in U.S. Pat. No. 5,051,833 (Tsuji). This patent describes an electronic still camera in which focus is based on a rectangular subset of the pixels on an interline charge coupled device (CCD) sensor. During the focusing operation, the image lines within the rectangle are read out slowly (regular video rates), while the lines outside of the rectangle are read out more rapidly to a charge drain on the CCD sensor. Tsuji thus describes a focusing mode which takes less time for reading out an image for the purpose of focusing than is taken for reading out an image for normal image capture. This focusing mode is employed with an interline transfer sensor, which has light-protected storage areas (vertical registers) and consequently does not require a mechanical shutter. Because the focusing image frames are not usable as captured images, this technique is not useful with camcorders. Moreover, focusing time can remain a problem if rapid utilization of the camera is desired or if longer readout times are required, such as for definitions higher than NTSC resolution.
With a full frame image sensor, it is commonly understood that mechanical shuttering is required to prevent vertical smear when reading out the sensor image, whether for focusing or normal imaging. This is particularly true for a high resolution full frame sensor having a relatively long readout time. Therefore, a mechanical shutter would need to be repeatedly opened and closed numerous times while capturing a sequence of images and focusing the lens. Therefore, full frame CCDs have not been used to perform autofocus in any known commercial electronic cameras. What is needed is a technique for rapidly focusing the lens of an electronic still camera which overcomes the problems just described.